Gemmules, also called plastitudes or pangenes, were assumed to be shed by the organs of the body and carried in the bloodstream to the reproductive organs where they accumulated in the germ cells or gametes. They thus provided a possible mechanism for the inheritance of acquired characteristics, as proposed by Jean-Baptiste Lamarck, which Darwin believed to be a cause of the observed variation in living organisms.

(from The Variation of Plants and Animals under Domestication (1868), Charles Darwin)

It is universally admitted that the cells or units of the body increase by self-division, or proliferation, retaining the same nature, and that they ultimately become converted into the various tissues and substances of the body. But besides this means of increase I assume that the units throw off minute granules which are dispersed throughout the whole system; that these, when supplied with proper nutriment, multiply by self-division, and are ultimately developed into units like those from which they were originally derived. These granules may be called gemmules. They are collected from all parts of the system to constitute the sexual elements, and their development in the next generation forms the new being; but they are likewise capable of transmission in a dormant state to future generations and may then be developed.)

Individual gemmules did not contain a complete microscopic blueprint for an entire creature in the way that Herbert Spencer or Carl von Nägeli described.' (p.276)

Pangenesis looked to him as if it might supply the answer. Darwin proposed that some limited effects from the environment might become embedded in an individual’s constitution and thus be liable to be transmitted, via the gemmules, to the offspring. (p.281)

But Darwin now wanted to include in his scheme the possibility of the inheritance of some limited acquired characteristics. Pangenesis gave him the chance to be Lamarckian without any of Lamarck’s inner strivings. As he put it, some aspects of the external environment could modify the inheritable gemmules.

In variations caused by the direct actions of changed conditions, of which several instances have been given, certain parts of the body are directly affected by the new conditions, and consequently throw off modified gemmules, which are transmitted to the offspring.18

No doubt the whole hypothesis of pangenesis was extremely complicated, he conceded. “But so are the facts.” (p.283–284)

Galton was troubled because he began the work in good faith, intending to prove Darwin right; and he praised pangenesis in Hereditary Genius in 1869. Somehow he had unintentionally proved Darwin wrong. Cautiously, he criticised his cousin’s theory, although qualifying his remarks by saying that Darwin’s gemmules (he called them “pangenes”) might be only temporary inhabitants of the blood and that his experiments could have failed to pick them up. (p.291–292)